ipconfig: Protect ic_addrservaddr with IPCONFIG_DYNAMIC.

>> net/ipv4/ipconfig.c:130:15: warning: 'ic_addrservaddr' defined but not used [-Wunused-variable]
    static __be32 ic_addrservaddr = NONE; /* IP Address of the IP addresses'server */

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>